Fuel cell, electronic device, movable body, power generation system, and cogeneration system

ABSTRACT

A fuel cell which can directly extract electric power from a polysaccharide, such as starch, is provided. A fuel electrode is formed by immobilizing with an immobilizer, on an electrode comprised of, e.g., carbon, an enzyme responsible for decomposing a polysaccharide into monosaccharides, an enzyme responsible for decomposing the monosaccharide formed, a coenzyme (e.g., NAD −  or NADP + ) which forms a reductant due to the oxidation reaction in the monosaccharide decomposition process, a coenzyme oxidase (e.g., diaphorase) for oxidizing the reductant of the coenzyme (e.g., NADH or NADPH), and an electron mediator (e.g., ACNQ or vitamin K3) for receiving electrons generated due to the oxidation of the coenzyme from the coenzyme oxidase and delivering the electrons to the electrode. The fuel cell comprises the fuel electrode and the air electrode that sandwich an electrolyte layer.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application is a Continuation of U.S. application Ser. No.11/570,113 filed on Mar. 13, 2007, which is a National Stage ofInternational Application No. PCT/JP05/10415 filed on Jun. 7, 2005 andwhich claims priority to Japanese Patent Application Nos. JP 2004-168430filed on Jun. 7, 2004 and JP 2005-159092 filed on May 31, 2005, theentire contents of which are being incorporated herein by reference.

BACKGROUND

A fuel cell basically comprises a fuel electrode (negative electrode),an oxidizer electrode or air electrode (positive electrode), and anelectrolyte (proton conductor), and has an operational principle, inaccordance with a reverse reaction of the electrolysis of water, suchthat hydrogen and oxygen are reacted to form water (H₂O) and generateelectricity. Specifically, a fuel (hydrogen) supplied to the fuelelectrode is oxidized and divided into electrons and protons (H⁺), andthe electrons go to the fuel electrode and the protons H⁺ travel throughthe electrolyte to the oxidizer electrode. At the oxidizer electrode,the protons H⁺ are reacted with oxygen supplied from the outside andelectrons fed from the fuel electrode through an external circuit toform H₂O.

The fuel cell is a high-efficient power generator which directlyconverts the chemical energy of a fuel to electrical energy, and canextract electrical energy from the chemical energy of fossil energy,such as natural gas, petroleum, or coal, with high conversionefficiency, irrespective of where or when the fuel cell is used. Forthis reason, conventionally, fuel cells in large-scale power generationapplications and others have been extensively researched and developed.For example, fuel cells are mounted on a space shuttle, which hasdemonstrated that the fuel cells can supply not only electric power butalso water for a crew, and that the fuel cells are clean powergenerators.

Further, in recent years, fuel cells operating in a range of relativelylow temperatures of from room temperature to about 90° C., such as solidpolymer fuel cells, are developed and have attracted attention.Therefore, attempts are being made to apply the fuel cell not only tothe large-scale power generation but also to small-size systems, such asa power source for driving an automobile, and a portable power sourcefor personal computer or mobile device.

As mentioned above, the fuel cell is possibly applied to a wide range ofuses from the large-scale power generation to the small-scale powergeneration, and has attracted considerable attention as a high-efficientpower generator. However, the fuel cell has various problems in that thefuel cell generally uses, as a fuel, hydrogen gas converted by means ofa reformer from natural gas, petroleum, or coal, and hence consumeslimited resources and requires high-temperature heating, and that thefuel cell needs a catalyst comprised of an expensive noble metal, suchas platinum (Pt). In addition, when hydrogen gas or methanol itself isdirectly used as a fuel, it must be carefully handled.

For solving the problems, the application of biological metabolismproceeding in a living body, which is a high-efficient energy conversionmechanism, to a fuel cell has been proposed. The biological metabolismused here includes respiration, photosynthesis, and the like conductedin microorganism somatic cells. The biological metabolism hasadvantageous features not only in that the power generation efficiencyis extremely high, but also in that the reaction proceeds under mildconditions at about room temperature.

For example, respiration is a mechanism such that microorganisms orcells take in nutrients, such as saccharides, fat, and protein, and,during the formation of carbon dioxide (CO₂) through the glycolyticpathway and tricarboxylic acid (TCA) cycle having a number of enzymereaction steps, nicotinamide adenine dinucleotide (NAD⁺) is reduced toform reduced nicotinamide adenine dinucleotide (NADH), thus convertingthe chemical energy of the nutrients to redox energy, i.e., electricalenergy, and further, in the electron transport system, the electricalenergy of NADH is directly converted to proton-gradient electricalenergy and oxygen is reduced to form water. The resultant electricalenergy forms ATP from adenosine diphosphate (ADP) through adenosinetriphosphate (ATP) synthase, and ATP is utilized in the reactionsrequired for living of microorganisms or cells. This energy conversionis carried out in cytosol and mitochondria.

Photosynthesis is a mechanism such that, during the conversion of theoptical energy taken in to electrical energy by reducing nicotinamideadenine dinucleotide phosphate (NADP +) through the electron transportsystem to reduced nicotinamide adenine dinucleotide phosphate (NADPH),water is oxidized to form oxygen. The resultant electrical energy isutilized in a carbon fixing reaction by taking in CO₂ and a synthesis ofcarbohydrates.

As a technique for utilizing the above-mention biological metabolism ina fuel cell, a microorganism battery in which electrical energygenerated in microorganisms is removed from the microorganisms throughan electron mediator and the resultant electrons are delivered to anelectrode to obtain an electric current has been reported (see, forexample, Unexamined Japanese Patent Application Laid-Open SpecificationNo. 2000-133297).

However, microorganisms and cells have many functions other than thedesired reactions including the conversion of chemical energy toelectrical energy. Therefore, in the above method, an undesired reactionconsumes the electrical energy, making it difficult to achieve asatisfactory energy conversion efficiency.

For solving the problem, a fuel cell in which only a desired reaction isadvanced using an enzyme and an electron mediator has been proposed(see, for example, Japanese Patent Application Publication Nos.2003-282124 and 2004-71559). In this fuel cell, a fuel is decomposedinto protons and electrons by an enzyme, and fuel cells using as a fuelan alcohol, such as methanol or ethanol, or a monosaccharide, such asglucose, have been developed.

However, the above-mentioned conventional fuel cell using alcohol orglucose as a fuel is unsatisfactory in power generation efficiency, andhence is difficult to put into practical use.

Accordingly, a task to be achieved by the present invention is toprovide a fuel cell which is advantageous not only in that it candirectly extract electric power from a polysaccharide to achievehigh-efficient power generation, but also in that it does not requirelimited fossil fuel and contributes to the realization of resourcecirculation society.

Another task to be achieved by the present invention is to provide anelectronic device, a movable body, a power generation system, and acogeneration system using the above excellent fuel cell.

SUMMARY

The present inventors have conducted extensive and intensive studieswith a view toward solving the above-mentioned problems accompanying theprior art technique. The studies are briefly described below.

Glucose used as a fuel in the above-mentioned conventional fuel cell isproduced by industrially decomposing a variety of polysaccharides. Onthe other hand, in the natural world, substances in the form ofmonosaccharides including glucose are not present, and many substancesare present in the form of polysaccharides. Generally, organisms do notobtain energy from glucose, and actually obtain energy by taking inpolysaccharides and decomposing them by an enzyme. Such extraction ofenergy from polysaccharides has been realized in a biomass system andthe like using, e.g., garbage as a fuel. This system produces a chemicalsubstance, such as hydrogen gas or methanol, by biomass, and many of thechemical substances produced provide heat energy by burning them.Further, heat generated by biological actions is utilized to obtain heatenergy. The heat energy is converted to kinetic energy using a turbineor the like, and further converted to electrical energy by a powergenerator. On the stages of energy conversion, energy loss occurs andthe energy of the fuel is considerably wasted.

Therefore, when a fuel cell that uses polysaccharides present in thenatural world as a fuel to generate electric power can be realized,electrical energy can be directly extracted from not only garbage butalso chemical substances produced in the natural world (e.g., starch andcellulose), thus making it possible to obtain electric power withoutusing limited fossil fuel. Further, garbage or waste paper can beeffectively utilized and hence the amount of waste is reduced, whichcontributes to the realization of resource circulation society.Furthermore, plants fix CO₂ in air in the synthesis of polysaccharidesby photosynthesis, which possibly contributes to the reduction of CO₂ inair which is a current problem.

The present inventors have made extensive and intensive studies. As aresult, it has been found that the use of a polysaccharide, such asstarch, as a fuel in the fuel cell can solve all the above problems, andthe present invention has been completed.

Specifically, for solving the above problems, the first invention in anembodiment is directed to a fuel cell for generating electric power bydecomposing a fuel using an enzyme, characterized in that the fuelcontains a polysaccharide.

The second invention in an embodiment is directed to an electronicdevice using a fuel cell, characterized in that the fuel cell generateselectric power by decomposing a fuel using an enzyme, wherein the fuelcontains a polysaccharide.

The third invention in an embodiment is directed to a movable body usinga fuel cell, characterized in that the fuel cell generates electricpower by decomposing a fuel using an enzyme, wherein the fuel contains apolysaccharide.

The fourth invention in an embodiment is directed to a power generationsystem using a fuel cell, characterized in that the fuel cell generateselectric power by decomposing a fuel using an enzyme, wherein the fuelcontains a polysaccharide.

The fifth invention in an embodiment is directed to a cogenerationsystem using a fuel cell, characterized in that the fuel cell generateselectric power by decomposing a fuel using an enzyme, wherein the fuelcontains a polysaccharide.

The fuel cell in each of the first to fifth inventions generally has astructure comprising a positive electrode and a negative electrode thatsandwich a proton conductor.

In the fuel cell, by decomposing a polysaccharide using an enzyme,electrical energy can be directly extracted from the polysaccharide.

Examples of polysaccharides usable as a fuel in the fuel cell(polysaccharides in a broad sense, meaning any carbohydrates that formtwo molecules or more of monosaccharides by hydrolysis, and includingoligosaccharides, such as disaccharides, trisaccharides, andtetrasaccharides) include starch, amylose, amylopectin, glycogen,cellulose, maltose, sucrose, and lactose. These comprise two or moremonosaccharides bonded together, and each polysaccharide containsglucose as a monosaccharide which is a bonding unit. Amylose andamylopectin are components of starch, and starch is a mixture of amyloseand amylopectin. As the fuel, any fuel can be used as long as itcontains a decomposable polysaccharide, and the fuel may contain glucosewhich is a decomposition product of a polysaccharide. Therefore, garbageor the like can be used as a fuel.

In the fuel cell, as the enzyme, at least a decomposing enzyme forpromoting decomposition, e.g., hydrolysis of the polysaccharide to forma monosaccharide, e.g., glucose and an oxidase for promoting oxidationof the monosaccharide formed to decompose it are used. Further, acoenzyme oxidase for changing a coenzyme reduced by the oxidase to anoxidant is also used.

When the reduced coenzyme is changed to an oxidant due to the action ofthe coenzyme oxidase, electrons are generated, and the electrons aredelivered from the coenzyme oxidase to an electrode (negative electrode)through an electron mediator. As the coenzyme, for example, NAD⁺ isused, and, as the coenzyme oxidase, for example, diaphorase is used.

In the fuel cell using glucoamylase as the decomposing enzyme fordecomposing a polysaccharide and using glucose dehydrogenase as theoxidase for decomposing a monosaccharide, a polysaccharide decomposableinto glucose by glucoamylase, for example, a substance comprising anyone of starch, amylose, amylopectin, glycogen, and maltose can be usedas a fuel to generate electric power. Glucoamylase is a decomposingenzyme that hydrolyzes an α-glucan, such as starch, to form glucose, andglucose dehydrogenase is an oxidase that oxidizes β-D-glucose toD-glucono-δ-lactone.

In the fuel cell using cellulase as the decomposing enzyme and usingglucose dehydrogenase as the oxidase, cellulose decomposable intoglucose by cellulase can be used as a fuel. Cellulase is, morespecifically, at least one member selected from cellulase (EC 3.2.1.4),exocellobiohydrase (EC 3.2.1.91), and β-glucosidase (EC 3.2.1.21). Asthe decomposing enzyme, a mixture of glucoamylase and cellulase may beused, and, in this case, the decomposing enzyme can decompose almost allthe naturally occurring polysaccharides, and therefore a powergeneration system using as a fuel a material containing a large amountof polysaccharides, for example, garbage can be achieved.

In the fuel cell using α-glucosidase as the decomposing enzyme and usingglucose dehydrogenase as the oxidase, maltose decomposable into glucoseby α-glucosidase can be used as a fuel.

In the fuel cell using sucrase as the decomposing enzyme and usingglucose dehydrogenase as the oxidase, sucrose decomposable into glucoseand fructose by sucrase can be used as a fuel. Sucrase is, morespecifically, at least one member selected from α-glucosidase (EC3.2.1.20), sucrose-α-glucosidase (EC 3.2.1.48), and β-fructofuranosidase(EC 3.2.1.26).

In the fuel cell using β-galactosidase as the decomposing enzyme andusing glucose dehydrogenase as the oxidase, lactose decomposable intoglucose and galactose by β-galactosidase can be used as a fuel.

For efficiently capturing the enzyme reaction phenomenon occurring nearthe negative electrode as an electric signal, it is preferred that thecoenzyme oxidase, coenzyme, and electron mediator are immobilized on thenegative electrode using an immobilizer. It is preferred that theoxidase is also immobilized on the negative electrode. Further, thedecomposing enzyme for decomposing a polysaccharide may be immobilizedon the negative electrode, and the polysaccharide finally used as a fuelmay also be immobilized on the negative electrode.

In the fuel cell using starch as a fuel, a gelatinized solid fuelobtained by gelatinizing starch can be used. In this case, there can beemployed a method in which gelatinized starch is brought into contactwith the negative electrode having immobilized thereon an enzyme andothers, or a method in which gelatinized starch is immobilized on thenegative electrode, together with an enzyme and others. When using suchan electrode, the starch concentration on the surface of the negativeelectrode can be kept high, as compared to that achieved when usingstarch in the form of a solution. Therefore, the decomposition reactionby the enzyme is faster and hence the output is improved, and furtherthe solid fuel is easier to handle than a solution fuel, and cansimplify the fuel supplying system, and, in addition, the fuel cell canbe moved upside down and hence is very advantageously used in mobiledevices.

The fuel cell of the first invention can be used in any devices whichneed electric power and have any sizes, and can be used in, for example,electronic devices, movable bodies, power devices, constructionmachines, machine tools, power generation systems, and cogenerationsystems, and the application of the fuel cell determines the output,size, or form of the fuel cell or the type of the fuel.

The electronic device of the second invention may be basically anyelectronic device, and involves both an electronic device of a portabletype and an electronic device of a fixed type, and, as specificexamples, there can be mentioned cellular phones, mobile devices,robots, personal computers, game machines, devices mounted on car,household appliances, and industrial products.

The movable body of the third invention may be basically any movablebody, and specific examples include automobiles, two-wheeled vehicles,aircrafts, rockets, and spacecrafts.

The power generation system of the fourth invention may be basically anypower generation system and may be on either a large scale or a smallscale, and, as a fuel, a polysaccharide, garbage comprisingpolysaccharides, or the like can be used.

The cogeneration system of the fifth invention may be basically anycogeneration system and may be on either a large scale or a small scale,and, as a fuel, a polysaccharide, garbage comprising polysaccharides, orthe like can be used.

In the present invention having the above-mentioned construction, apolysaccharide contained in the fuel is decomposed by an enzyme intomonosaccharides, and electrical energy can be efficiently extracted uponoxidizing the monosaccharides by an enzyme.

In the present invention, an enzyme is used as a catalyst and a materialcomprising a polysaccharide is used as a fuel, and thus a fuel cellcapable of generating electric power with high efficiency using apolysaccharide as a fuel can be obtained. The fuel cell makes itpossible to directly extract electrical energy from, for example,garbage containing a large amount of polysaccharides, thus enablingeffective utilization of garbage. Further, the fuel cell does notrequire limited fossil fuel, and contributes to the realization ofresource circulation society. In addition, a polysaccharide which issafe when it is eaten can be used as a fuel, and therefore a fuel celladvantageously used as a mobile product can be obtained. Furthermore,for example, when using gelatinized starch as a fuel, the fuel is easierto handle than a solution fuel, and can simplify the fuel supplyingsystem, thus obtaining a fuel cell advantageously used as a mobileproduct. The use of this excellent fuel cell can realize ahigh-performance electronic device, movable body, power generationsystem, or cogeneration system.

Additional features and advantages are described herein, and will beapparent from the following Detailed Description and the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic diagram showing the construction of a fuel cellaccording to one embodiment of the present invention.

FIG. 2 is a schematic diagram illustrating the decomposition of starchand cellulose into glucose by an enzyme.

FIG. 3 is a schematic diagram showing the construction of a fuel cellaccording to one embodiment of the present invention using starch glue.

FIG. 4 are diagrams showing the glucose concentration distribution inthe direction perpendicular to the surface of the fuel electrode in thefuel cell according to one embodiment of the present invention.

FIG. 5 are diagrammatic views showing a fuel cartridge used in the fuelcell according to one embodiment of the present invention.

FIG. 6 is a diagrammatic view for explaining a method for supplying afuel to the fuel cell according to one embodiment of the presentinvention.

FIG. 7 is a diagrammatic view for explaining a method for supplying afuel to the fuel cell according to one embodiment of the presentinvention.

FIG. 8 is a diagrammatic view for explaining an example of the methodfor supplying a fuel to the fuel cell according to one embodiment of thepresent invention.

FIG. 9 is a diagrammatic view for explaining another example of themethod for supplying a fuel to the fuel cell according to one embodimentof the present invention.

FIG. 10 is a diagram showing the results of the CV measurement inExample 1 of the present invention.

FIG. 11 is a diagram showing a change of the current density with timeat a constant potential 0 V of the working electrode relative to thereference electrode in the electrochemical measurements in Examples 1and 2 of the present invention.

FIG. 12 is a diagram showing the results of the CV measurement inExample 3 of the present invention.

FIG. 13 are diagrammatic views of a fuel cell used in the evaluation inExample 4 of the present invention.

FIG. 14 is a diagram showing the results of the measurement ofcurrent-voltage characteristics of the fuel cell used in the evaluationin Example 4 of the present invention.

DETAILED DESCRIPTION

Hereinbelow, an embodiment of the present invention will be describedwith reference to the accompanying drawings.

FIG. 1 diagrammatically shows a fuel cell according to one embodiment ofthe present invention. As shown in FIG. 1, the fuel cell comprises afuel electrode (negative electrode) 1 which decomposes a polysaccharidesupplied as a fuel using an enzyme to get electrons and generate protons(H⁺), an electrolyte layer 3 which conducts only protons, and an airelectrode (positive electrode) 5 which is separated from the fuelelectrode 1 by the electrolyte layer 3, and which forms water fromprotons transported from the fuel electrode 1 through the electrolytelayer 3, electrons fed from the fuel electrode 1 through an externalcircuit, and oxygen in air.

The fuel electrode 1 comprises an electrode 11 comprised of, e.g.,carbon having immobilized thereon, by an immobilizer (e.g., a polymer),an enzyme responsible for decomposing a polysaccharide intomonosaccharides, an enzyme responsible for decomposing themonosaccharide formed, a coenzyme (e.g., NAD⁺ or NADP⁺) which forms areductant due to the oxidation reaction in the monosaccharidedecomposition process, a coenzyme oxidase (e.g., diaphorase) foroxidizing the reductant (e.g., NADH or NADPH) of the coenzyme, and anelectron mediator (e.g., 2-amino-3-carboxy-1,4-naphthoquinone; ACNQ, orvitamin K3) for receiving electrons generated due to the oxidation ofthe coenzyme from the coenzyme oxidase and delivering the electrons tothe electrode 11.

The polysaccharide usable as a fuel comprises two or moremonosaccharides bonded together. Examples of the polysaccharides includedisaccharides, such as maltose, sucrose, and lactose, starch comprisingamylose having a linear molecule and amylopectin having a branchedmolecule, high-molecular glycogen having an amylopectin-like branchedmolecule, cellulose having a linear molecule, and saccharides asintermediates thereof.

As an enzyme responsible for decomposing a polysaccharide, a decomposingenzyme capable of cutting a glycoside linkage by hydrolysis or the likeis used. When the polysaccharide is starch, examples of the decomposingenzymes include hydrolases, such as glucoamylase (EC 3.2.1.3), α-amylase(EC 3.2.1.1), and β-amylase (EC 3.2.1.2). Among these, glucoamylase(GAL) decomposes starch into glucose. EC indicates an enzyme number.

Other polysaccharides can be decomposed by, for example, the followingdecomposing enzymes.

<Glycogen>

-   -   Glucoamylase (EC 3.2.1.3)    -   α-Amylase (EC 3.2.1.1)

<Dextrin>

-   -   Glucoamylase (EC 3.2.1.3)

<Cellulose>

-   -   Cellulase (EC 3.2.1.4)    -   Exocellobiohydrase (EC 3.2.1.91)    -   β-Glucosidase (EC 3.2.1.21)

Here, enzymes capable of hydrolyzing cellulose are collectively referredto as “cellulase”. As examples of cellulases, there can be mentioned theabove three types of enzymes, and cellulose can be decomposed intoglucose in the presence of at least one member selected from the threeenzymes.

<Maltose>

-   -   α-Glucosidase (EC 3.2.1.20)

This enzyme is also called maltase, but it acts on sucrose. Maltose canbe decomposed also by glucoamylase.

<Sucrose>

-   -   α-Glucosidase (EC 3.2.1.20)    -   Sucrose-α-glucosidase (EC 3.2.1.48)    -   β-Fructofuranosidase (EC 3.2.1.26)

Here, enzymes capable of hydrolyzing sucrose are collectively referredto as “sucrase”. As examples of sucrases, there can be mentioned theabove three types of enzymes. Glucose can be formed from sucrose in thepresence of at least one member selected from the three enzymes.

<Lactose>

-   -   β-Galactosidase (EC 3.2.1.23)

<1,3-β-Glucan>

-   -   Glucanendo-1,3-β-D-glucosidase (EC 3.2.1.39)

<α,α-Trehalose>

-   -   α,α-Trehalase (EC 3.2.1.28)    -   α,α-Trehalose phosphorylase (EC 2.4.1.64)

<Stachyose>

-   -   α-Galactosidase (EC 3.2.1.22)    -   α-Glucosidase (EC 3.2.1.20)

<Glucosides>

-   -   β-Glucosidase (EC 3.2.1.21)

FIG. 2 shows an example of the decomposition of starch and celluloseinto glucose by an enzyme.

The enzyme responsible for decomposing a monosaccharide includes anoxidase that participates in the redox reaction in the decompositionprocess. When the polysaccharide is starch, glycogen, cellulose, ormaltose, the monosaccharide formed by hydrolysis of the polysaccharideis glucose, and, when the polysaccharide is sucrose or lactose, glucoseconstitutes half of the monosaccharide formed. As an example of anenzyme responsible for decomposing glucose, there can be mentionedglucose dehydrogenase (GDH). The use of this oxidase can oxidizeβ-D-glucose to D-glucono-δ-lactone.

Further, the resultant D-glucono-δ-lactone can be decomposed into2-keto-6-phospho-D-gluconate in the presence of two enzymes, i.e.,gluconokinase and phosphogluconate dehydrogenase (PhGDH). Specifically,D-glucono-δ-lactone is hydrolyzed into D-gluconate, and D-gluconate isphosphorylated by hydrolyzing adenosine triphosphate (ATP) in thepresence of gluconokinase into adenosine diphosphate (ADP) andphosphoric acid, thus forming 6-phospho-D-gluconate. The resultant6-phospho-D-gluconate is oxidized to 2-keto-6-phospho-D-gluconate due tothe action of oxidase PhGDH.

In addition to the above decomposition process, glucose can bedecomposed into CO₂ utilizing carbohydrate metabolism. The decompositionprocess utilizing carbohydrate metabolism is roughly classified intodecomposition of glucose and formation of pyruvic acid through aglycolytic pathway and a TCA cycle, and these are widely known reactionsystems.

The oxidation reaction in the monosaccharide decomposition processaccompanies a reduction reaction of a coenzyme. The coenzyme isdetermined almost always depending on the enzyme used, and, when theenzyme is GDH, NAD⁺ is used as a coenzyme. Specifically, when GDH causesβ-D-glucose to be oxidized to D-glucono-δ-lactone, NAD⁺ is reduced toNADH to generate H⁺.

The resultant NADH is immediately oxidized to NAD⁺ in the presence ofdiaphorase (DI) to form two electrons and two protons H⁺. In otherwords, in the oxidation reaction on the first stage, two electrons andtwo protons H⁺ are formed per one molecule of glucose. In the oxidationreaction on the second stage, four electrons and four protons H⁺ intotal are formed.

The electrons generated in the above process are delivered fromdiaphorase to the electrode 11 through an electron mediator, and theprotons H⁺ are transported to the air electrode 5 through theelectrolyte layer 3.

The electron mediator delivers electrons to the electrode, and thevoltage of the fuel cell depends on the oxidation-reduction potential ofthe electron mediator. In other words, for obtaining a higher voltage,an electron mediator having a more negative potential on the fuelelectrode 1 side is preferably selected, but the reaction affinity ofthe electron mediator with an enzyme, the electron exchange rate for theelectrode 11, the structure stability of the electron mediator to aninhibition factor (such as light or oxygen), and the like must be takeninto consideration. From this point of view, as an electron mediator forthe fuel electrode 1, for example, ACNQ or vitamin K3 is preferred.Alternatively, a compound having, e.g., a quinone skeleton, a metalcomplex of Os, Ru, Fe, or Co, a violegen compound, such as benzylviolegen, a compound having a nicotinamide structure, a compound havinga riboflavin structure, or a compound having a nucleotide-phosphatestructure can be used as an electron mediator.

For achieving an efficient and steady electrode reaction, it ispreferred that the enzyme, coenzyme, and electron mediator aremaintained at pH optimum for the enzyme, for example, pH about 7, usinga buffer, such as a Tris buffer or a phosphate buffer. Further, toolarge or too small ion strength (I.S.) adversely affects the enzymeactivity, and, from the viewpoint of achieving excellent electrochemicalresponse, it is preferred that the ion strength is an appropriate value,for example, about 0.3. The enzymes used individually have the optimumpH and the optimum ion strength, and hence the pH and ion strength arenot limited to the above values.

The enzyme, coenzyme, and electron mediator may be used in the form of asolution in a buffer, but, for efficiently capturing the enzyme reactionphenomenon occurring near the electrode as an electric signal, it ispreferred that at least the coenzyme oxidase and the electron mediatorare immobilized on the electrode 11 using an immobilizer. When theenzyme for decomposing a fuel and the coenzyme are further immobilizedon the electrode 11, the enzyme reaction system at the fuel electrode 1can be stabilized. As the immobilizer, for example, glutaraldehyde (GA)and poly-L-lysine (PLL) can be used in combination. They may be usedindividually, or other polymers may be used. By using an immobilizercomprising a combination of glutaraldehyde and poly-L-lysine, the enzymeimmobilizing abilities of the individual components can be fullyutilized, so that the immobilizer exhibits collectively excellent enzymeimmobilizing ability. In this case, an optimum ratio of glutaraldehydeto poly-L-lysine varies depending on the enzyme to be immobilized and asubstrate of the enzyme, but, generally, the ratio may be arbitrary.Specifically, a ratio of an aqueous glutaraldehyde solution (0.125%) toan aqueous poly-L-lysine solution (1%) is 1:1, 1:2, or 2:1.

FIG. 1 shows an example in which the polysaccharide is starch, theenzyme responsible for decomposing the polysaccharide intomonosaccharides is glucoamylase (GAL) which decomposes starch intoglucose, the enzyme responsible for decomposing the monosaccharideformed (β-D-glucose) is glucose dehydrogenase (GDH), the coenzyme whichforms a reductant due to the oxidation reaction in the monosaccharidedecomposition process is NAD⁺, the coenzyme oxidase for oxidizing NADHwhich is the reductant of the coenzyme is diaphorase (DI), and theelectron mediator for receiving from the coenzyme oxidase electronsgenerated due to the oxidation of the coenzyme and delivering theelectrons to the electrode 11 is ACNQ.

The electrolyte layer 3 is comprised of a material which is aproton-conductive membrane for transporting protons H⁺ generated at thefuel electrode 1 to the air electrode 5 and which has no electronconduction properties and can transport protons H⁺. Examples of theelectrolyte layers 3 include layers comprised of aperfluorocarbonsulfonic acid (PFS) resin membrane, a trifluorostyrenederivative copolymer membrane, a polybenzimidazole membrane impregnatedwith phosphoric acid, an aromatic polyether ketone sulfonic acidmembrane, PSSA-PVA (polystyrenesulfonic acid-polyvinyl alcoholcopolymer), or PSSA-EVOH (polystyrenesulfonic acid-ethylenevinyl alcoholcopolymer). Of these, preferred is a layer comprised of an ion-exchangeresin having a fluorine-containing carbon sulfonic acid group, and,specifically, Nafion (trade name; manufactured and sold by Du Pont Co.,U.S.A.) is used.

The air electrode 5 is comprised of carbon powder having a catalystcarried thereon or catalyst particles which are not supported on carbon.In the catalyst, for example, fine particles of platinum (Pt), or fineparticles of an alloy of a transition metal, such as iron (Fe), nickel(Ni), cobalt (Co), or ruthenium (Ru), and platinum or an oxide are used.The air electrode 5 has a structure such that, for example, a catalystor a catalyst layer comprised of carbon powder comprising a catalyst anda gas diffusion layer comprised of a porous carbon material are stackedin this order from the electrolyte layer 3 side. The structure of theair electrode 5 is not limited to this, and an oxidoreductase can beused as a catalyst. In this case, the oxidoreductase and an electronmediator for delivering electrons to the electrode are used incombination.

At the air electrode 5, protons H⁺ from the electrolyte layer 3 andelectrons from the fuel electrode 1 reduce oxygen in air in the presenceof a catalyst to form water.

In the fuel cell having the above-described construction, when apolysaccharide, such as starch, is supplied to the fuel electrode 1, thepolysaccharide is hydrolyzed by an enzyme into monosaccharides, such asglucose, and further the monosaccharide is decomposed by a decomposingenzyme comprising an oxidase. The oxidase participates in themonosaccharide decomposition process to form both electrons and protonsH⁺ on the fuel electrode 1 side, making it possible to generate anelectric current between the fuel electrode 1 and the air electrode 5.

In the fuel cell, the type of the decomposable polysaccharide isdetermined depending on the type of the decomposing enzyme held orimmobilized on the fuel electrode 1. When a mixture containing aplurality of polysaccharides is used as a fuel, enzymes for respectivelydecomposing the polysaccharides are held or immobilized on the fuelelectrode 1, thus improving the fuel efficiency. In addition, garbage orthe like can be used as a fuel to generate electric power, enablingeffective utilization of garbage and others.

Furthermore, the above fuel cell can use as a fuel a polysaccharidehaving a high energy density, which is safe when it is eaten, andfurther can work at room temperature, and therefore is advantageouslyused in mobile products. With respect to the energy density obtainedwhen using a polysaccharide as a fuel, taking cooked white rice as anexample, the energy of starch contained in about 100 g of cooked rice(corresponding to one rice bowl and about 160 kcal) corresponds to theenergy of 64 AA-size alkali dry cells (about 3 Wh per cell), which is ahigh fuel energy density equal to or more than that obtained from aglucose solution. Polysaccharide as a fuel can be used in the form of anaqueous solution, but there can be employed a method in which thepolysaccharide is gelatinized like starch glue and brought into contactwith the fuel electrode 1 or a method in which the polysaccharide isdisposed within the fuel electrode 1, and thus a solidified fuel can beused in the fuel cell, which is further advantageous to mobile products.Glucose has a problem in that glucose having a small diffusioncoefficient, as compared to methanol or ethanol, is disadvantageous to afuel-molecule diffusion controlled reaction which proceeds when theglucose is used as a fuel in the form of a solution. However, when amethod in which starch is gelatinized and brought into contact with thefuel electrode 1 or a method in which starch is disposed within the fuelelectrode 1 is employed, the starch concentration on the surface of thefuel electrode 1 or in the fuel electrode 1, namely, the glucoseconcentration can be kept high, so that the output is improved, ascompared to that achieved when using starch in the form of a solution.In addition, a solidified fuel, such as starch glue, is easier to handlethan a liquid fuel, and can simplify the fuel supplying system, and thusit is very effective when the fuel cell is applied to mobile products.FIG. 3 shows an example in which starch glue 6 as a fuel is immobilizedon the fuel electrode 1.

When starch which is a polysaccharide is used as a fuel, the glucoseconcentration on the surface of the fuel electrode 1 or in the fuelelectrode 1 can be kept high, as compared to that obtained when usingglucose which is a monosaccharide as a fuel. Specifically, for example,amylose contained in starch comprises several hundred to severalthousand molecules of glucose bonded together, and, when one molecule ofamylose as a fuel molecule arrives at the surface of the fuel electrode1 by diffusion, the transport speed of glucose to the surface of thefuel electrode 1 is several hundred to several thousand times thatachieved when using glucose as a fuel. In other words, the use of starchas a fuel makes it possible to transport glucose to the surface of thefuel electrode 1 at a higher speed.

FIG. 4A shows a state such that a CA measurement (in the measurement ofa change of the current with time at a constant potential, a state inwhich a steady current flows is a diffusion-controlled state) wasconducted in a solution containing starch and glucoamylase (GAL) usingan enzyme-immobilized electrode having ACNQ, diaphorase (DI), andglucose dehydrogenase (GDH) immobilized on the electrode 11 by animmobilizer and a satisfactorily long period of time had lapsed(diffusion-controlled state). Similarly, FIG. 4B shows a state such thata CA measurement was conducted in a glucose solution using the sameenzyme-immobilized electrode and a satisfactorily long period of timehad lapsed (diffusion-controlled state). The enzyme reaction in theenzyme-immobilized electrode is satisfactorily fast, that is, glucosewhich has reached the surface of the electrode by diffusion can be veryrapidly decomposed to deliver electrons to the electrode. In the case ofFIG. 4B, on the surface of the electrode, the consumption of glucose bythe enzyme-immobilized electrode balances with the supply of glucose bydiffusion from the glucose solution far away from the enzyme-immobilizedelectrode to exhibit a constant glucose concentration gradient. Thisglucose concentration gradient determines a current, and, the larger theglucose concentration gradient, the larger the current. That is, thecurrent can be increased by increasing the glucose concentration. On theother hand, in the case of FIG. 4A, no glucose is present in thesolution at the beginning of the measurement, but the solution containsglucoamylase as well as starch, and hence glucoamylase hydrolyzes starchto form glucose. In this case, on the surface of the electrode, theconsumption of glucose by the enzyme-immobilized electrode balances withthe supply by diffusion of glucose formed in the solution containingstarch and glucoamylase and the supply of glucose formed due toglucoamylase present on the surface of the electrode, determining acurrent. The glucose formed on the surface of the electrode increasesthe glucose concentration on the surface of the electrode, as comparedto that obtained when using a glucose solution (compared in terms of theultimate glucose saturated solution). When glucoamylase and starch arefurther immobilized on the enzyme-immobilized electrode, that is theconstruction shown in FIG. 3 is employed, the above effect can befurther improved.

Next, a method for supplying a fuel to the fuel cell is described. Here,the case where starch is used as a fuel is mentioned.

FIG. 5A shows an unused, card-shaped fuel cartridge 32 filled with afuel 31 comprising a starch solution (amylose, amylopectin), starchglue, or the like. The fuel 31 may contain glucose, NADH, or the like,and, in such a case, the current at the beginning of the operation canbe large, as compared to the current obtained when using a fuel 31comprising only starch. FIG. 5B shows the used fuel cartridge 32 whichhas used all the fuel 31. In FIGS. 5A and 5B, reference numerals 33 a,33 b designate fuel pushers. Reference numeral 33 c designates a springfor pushing the fuel, having both ends fixed to the fuel pushers 33 a,33 b. The fuel pusher 33 a is fixed to the fuel cartridge 32, and thespring 33 c pushes the fuel pusher 33 b against the fuel 31.

FIG. 6 shows the fuel cell in which the fuel cartridge 32 has used allthe fuel 31. The fuel cartridge 32 is contained in a fuel cartridgecontainer 34. The fuel cartridge container 34 has a cartridge inlet 34 athrough which the fuel cartridge 32 is inserted into the fuel cartridgecontainer and a cartridge outlet 34 b through which the fuel cartridge32 is removed from the container. The fuel cell has a construction suchthat an air electrode 5 comprised of an enzyme-immobilized carbonelectrode having an enzyme immobilized on porous carbon and a fuelelectrode 1 comprised of an enzyme-immobilized carbon electrode havingan enzyme and an electron mediator immobilized by an immobilizer onporous carbon like in Example 1 are disposed so that they face eachother through a separator 35 as a proton conductor (corresponding to theelectrolyte layer 3). In FIG. 6, as an example of a load of an externalcircuit, an electric bulb 36 is connected to the air electrode 5 and thefuel electrode 1. The fuel cartridge 32 has used all the fuel 31, andtherefore the electric bulb 36 is not lightening. The fuel cartridge 32generally has a size larger than that of the air electrode 5 or fuelelectrode 1.

The used fuel cartridge 32 is changed to an unused fuel cartridge 32 asfollows. As shown in FIG. 7, the cartridge inlet 34 a is opened, and theunused fuel cartridge 32 is inserted into the fuel cartridge container34 to push the used fuel cartridge 32, letting it go out of thecontainer through the cartridge outlet 34 b. At a point in time when theused fuel cartridge 32 is completely removed from the cartridge outlet34 b, the unused fuel cartridge 32 is set in a predetermined position.This state is shown in FIG. 8. As shown in FIG. 8, when the unused fuelcartridge 32 is set in the position, a feed passage for the fuel 31 isformed between the fuel cartridge 32 and the fuel electrode 1, so thatthe fuel 31 is supplied to the fuel electrode 1 through the feedpassage. This can be easily realized in an electromechanical system. Inthis instance, in the fuel cartridge 32, the fuel pusher 33 b pushes thefuel 31, and therefore the fuel 31 can be fed to the inside of the fuelelectrode 1 comprised of an enzyme-immobilized carbon electrode havingan enzyme and an electron mediator immobilized by an immobilizer onporous carbon. This is effective when, for example, liquid having a highviscosity is used as the fuel 31. When the fuel 31 can reach the insideof the fuel electrode 1 merely by diffusion, the fuel pushers 33 a, 33 band spring 33 c can be omitted, but, when using the fuel pushers 33 a,33 b and spring 33 c, the fuel 31 can be more surely supplied to theinside of the fuel electrode 1. Thus, the fuel 31 is supplied to thefuel electrode 5 to start power generation, so that the electric bulb 36lightens.

It is preferred that CO₂ or H₂O, or both generated in the powergeneration are stored in a vacant space of the fuel cartridge 32 leftafter all the fuel 31 has been used. Specifically, CO₂ or H₂O may bedischarged from the fuel cell, but, from an environmental point of view,more specifically, discharging CO₂ from the fuel cell is not preferredfrom the viewpoint of preventing global warming, and further,discharging H₂O from the fuel cell has a problem in that, when, e.g., acellular phone having the fuel cell mounted is put in a pocket or bag,the pocket or bag may be wetted with water discharged, and therefore itis preferred to store CO₂ or H₂O, or both in the fuel cartridge 32. CO₂or H₂O can be efficiently stored in a vacant space of the fuel cartridge32 left after all the fuel 31 has been used.

Amylase may be put in the portion of the fuel cartridge 32 containingthe fuel 31. In this case, the concentration of glucose supplied to thesurface of the fuel electrode 1 is increased, thus making it possible toobtain a large current and a large initial current.

As the fuel cartridge 32, a fuel cartridge preliminarily filled with thefuel 31 may be used, or, for emergency, the fuel cartridge 32 into whicheasily available cooked rice, pasta, potato, or the like, which isappropriately treated, is charged may be used. A method for charging thefuel 31 into the fuel cartridge 32 may be in which, for example, a fuelreservoir container is prepared and a fuel inlet formed in the fuelcartridge 32 is connected to the container to charge the fuel 31. Inthis instance, the fuel cartridge 32 may be either removed from the fuelcell or not.

With respect to the treatment of starch, starch in raw rice or potato iscomprised mainly of β-starch crystallites and amylase exhibits almost noactivity, but, when starch is heated, β-starch is changed to gelatinizedα-starch, so that amylase exhibits an activity. For this reason, it ispreferred to supply α-starch as the fuel 31 to the fuel electrode 1, butα-starch is changed to β-starch with the passage of time (aging).

The fuel 31 having moisture extremely reduced or having almost nomoisture can be used. Starch can be solidified by pressing it. Glucosecan be solidified by this method, but it has poor formability. In thesolidified fuel 31, molecules are unlikely to diffuse, and therefore thesolidified fuel cannot be used as it is. In this case, while contactingthe fuel 31 and the fuel electrode 1 with each other, water may besupplied from the outside or from the inside of the fuel cartridge 32(in which the starch solid material and water are separated from eachother), so that the fuel cell starts power generation. As the water,water formed at the air electrode 5 may be utilized based on theprinciple similar to that of a direct methanol fuel cell (DMFC) using a100% methanol fuel. This is a system in which the fuel electrode 1 andthe air electrode 5 collectively form water in principle. The reactionin this system is represented by the following formula:

C₆H₁₂O₆+6O₂→6CO₂+6H₂O ΔG°=−4,928 kJ/mol.

Next, a method for supplying a fuel to the fuel cell used as a throwawayprimary battery, such as a dry cell, is described.

In this case, a mechanism for removing and inserting the fuel cartridge32 to the fuel cell is not needed, and, as shown in FIG. 9, the fuelcartridge 32 is preliminarily unified with the fuel electrode 1. In thiscase, the method for supplying the fuel 31 from the fuel cartridge 32 tothe fuel electrode 1 is similar to the method described above.

Hereinbelow, the present invention will be described with reference tothe following Examples.

Example 1

Onto a glassy carbon (GC) electrode (BAS, φ=3.0 mm) were applieddropwise 3 μl of a phosphate buffer solution (83 μM) of diaphorase(DI)(UNITIKA LTD., from Bacillus stearothermophilus), 6 μl of aphosphate buffer solution (60 μM) of glucose dehydrogenase (GDH)(TOYOBOLTD.), 3 μl of a phosphate buffer solution (1.4 mM) of glucoamylase(GAL)(Oriental Yeast Co., Ltd.), 3 μl of an aqueous solution (1%) ofpoly-L-lysine (PLL), 2 μl of a phosphate buffer solution (0.4 M) ofNADH, 2 μl of an ethanol solution (28 mM) of ACNQ, and 3 μl of anaqueous solution (0.125%) of glutaraldehyde (GA), and they were mixedwith each other and air-dried at room temperature, followed by washingwith distilled water, preparing a GAL/GDH/NADH/DI/ACNQ-immobilizedelectrode (see FIG. 1).

The thus prepared immobilized electrode was used as a working electrode,an Ag/AgCl electrode was used as a reference electrode, a Pt electrodewas used as a counter electrode, an electrolytic cell made ofpolytetrafluoroethylene having a capacity of 1 ml was used as a reactionbath, and 1 ml of a 0.1 M phosphate buffer solution (pH: 7; I.S.=0.3)containing water-soluble starch in a concentration of 1% was used as areaction solution, and they were subjected to deoxygenation using Argas, and then an electrochemical measurement was conducted at roomtemperature (25° C.).

Example 2

An electrochemical measurement was conducted in substantially the samemanner as in Example 1 except that 5 mg of a material obtained bygelatinizing a 50% phosphate buffer solution of starch at 70° C. wasapplied to the GAL/GDH/NADH/DI/ACNQ-immobilized electrode prepared inExample 1, and that the reaction solution was changed to 1 ml of a 0.1 Mphosphate buffer solution (pH 7; I.S.=0.3).

Comparative Example 1

An electrochemical measurement was conducted in substantially the samemanner as in Example 1 except that glucoamylase (GAL) was omitted in theprocess of preparing the immobilized electrode in Example 1 to prepare aGDH/NADH/DI/ACNQ-immobilized electrode.

With respect to Example 1, potential scan was performed in accordancewith a potential sweep method (CV) at a scanning speed of 20 mV/sec. Theresult of the CV measurement is shown in FIG. 10 (solid line a in thegraph). In FIG. 10, for reference, the result of the CV measurement inthe case where a 0.1 M phosphate buffer solution containing no starchwas used as a reaction solution is shown by a broken line b, and theresult of the CV measurement in the case where a 0.1 M phosphate buffersolution containing starch in a concentration of 0.1% was used as areaction solution is shown by a dot-dash line c. In the measurement inthe case where a solution containing starch in a concentration of 0.5%was used, a result substantially the same as the solid line a wasobtained.

As seen from FIG. 10, when the reaction solution contains starch, anoxidation current is observed, which indicates that starch in themembrane immobilized on the electrode is decomposed by GAL into glucoseand the glucose is decomposed by GDH, and the reactions successivelyproceed, so that the electrode receives electrons.

In each of the electrochemical measurements in Examples 1 and 2 andComparative Example 1, a change of the current density with time at aconstant potential 0 V of the working electrode relative to thereference electrode is shown in FIG. 11. Further, steady currentsobtained after a satisfactorily long period of time has lapsed in FIG.11 are shown in Table 1.

TABLE 1 Steady current (μA/cm² @0 V vs Ag|AgCl) Example 1 58 Example 2395 Comparative Example 1 2

As can be seen from Table 1, in Examples 1 and 2 in which glucoamylasewhich is an enzyme decomposing starch was present, a current could begenerated using starch as a fuel. Especially in Example 2 in whichstarch was gelatinized and immobilized on the surface of the electrode,there could be obtained a current larger than that obtained in Example 1in which starch in the form of a solution was in contact with theelectrode. The reason for this is that the starch concentration, namely,glucose concentration on the surface of the electrode can be kept highas mentioned above, making it possible to promote the decompositionreaction of the fuel.

Glucose has a problem in that glucose having a small diffusioncoefficient, as compared to methanol or ethanol, is disadvantageous to adiffusion controlled reaction which is likely to proceed when theglucose is used as a fuel in the form of a solution, but, as apparentfrom the above results, the problem can be solved by using starch as afuel or immobilizing gelatinized starch on the surface of the electrode.Further, the use of a gelatinized solid fuel makes easy handling of thefuel and can simplify the fuel supplying system, and hence the resultantfuel cell is very useful as a fuel cell mounted on mobile devices, suchas cellular phones.

Example 3

With respect to the fuel electrode 1 having immobilized thereon starchglue 6 as a fuel and glucoamylase (GAL) as an enzyme decomposing starchinto glucose, a CV measurement was conducted under the same conditionsas those used in Example 1. The results are shown in FIG. 12 (curvedline a). In FIG. 12, for reference, the result of the CV measurement inthe case where a glucose solution was used as a fuel is also shown(curved line b). As can be seen from FIG. 12, when the starch glue 6 isused as a fuel, a considerably large current can be obtained, ascompared to the maximum current obtained when the glucose solution(glucose concentration: 200 mM) is used as a fuel. This result reflectsthe extremely high glucose concentration on the surface of the fuelelectrode 1 as mentioned above. Further, the reason why the currentincreases with the passage of time resides in that the starch isgradually hydrolyzed by glucoamylase (GAL) and, consequently, theglucose concentration on the surface of the electrode increases with thepassage of time. The curved line b has a shape unique to a diffusioncontrolled reaction.

Example 4

A fuel cell shown in FIGS. 13A and 13B was assembled and an evaluationwas made. As shown in FIGS. 13A and 13B, the fuel cell has aconstruction such that an air electrode 5 comprised of anenzyme-immobilized carbon electrode having an enzyme immobilized on 0.25cm² carbon felt and a fuel electrode 1 comprised of anenzyme-immobilized carbon electrode having an enzyme and an electronmediator immobilized by an immobilizer on 0.25 cm² carbon felt like inExample 1 are disposed so that they face each other through a separator35 as a proton conductor. In this case, the separator 35 is comprised ofa predetermined film having proton conduction properties, e.g.,cellophane. Ti current collectors 41, 42 are disposed, respectively,under the air electrode 5 and on the fuel electrode 1, thus facilitatingcurrent collection. Reference numerals 43, 44 designate clamp plates.The clamp plates 43, 44 are fastened to each other with screws 45, andbetween them are sandwiched whole of the air electrode 5, fuel electrode1, separator 35, and Ti current collectors 41, 42. A circular recessedportion 43 a for drawing air is formed in one side (outer side) of theclamp plate 43, and a number of holes 43 b are formed at the bottom ofthe recessed portion 43 a so that they penetrate the clamp plate 43 toanother side. The holes 43 b serve as air feed passages to the airelectrode 5. On the other hand, a circular recessed portion 44 a forcharging a fuel is formed in one side (outer side) of the clamp plate44, and a number of holes 44 b are formed at the bottom of the recessedportion 44 a so that they penetrate the clamp plate 44 to another side.The holes 44 b serve as fuel feed passages to the fuel electrode 1. Aspacer 46 is provided at the periphery portion of another side of theclamp plate 44 so that the clamp plates 43, 44 fastened with the screws45 have a predetermined space between them.

As shown in FIG. 13B, a load 47 was connected to the Ti currentcollectors 41, 42, and a starch/buffer solution was charged as a fuelinto the recessed portion 44 a in the clamp plate 44 to perform powergeneration. The working temperature was 25° C. FIG. 14 shows thecurrent-voltage characteristics of this fuel cell. The open circuitvoltage is about 0.86 V. In FIG. 14, a curved line a indicates a currentdensity, and a curved line b indicates a power density. As shown in FIG.14, the current density is about 1 mA/cm² at the maximum and the powerdensity is about 0.4 mW/cm² at the maximum, which indicates that bothvalues are high.

Hereinabove, one embodiment and Examples of the present invention aredescribed in detail, but the present invention is not limited to theabove embodiment or Examples and can be changed or modified based on thetechnical concept of the present invention.

For example, values, structures, constructions, forms, and materialsmentioned in the above embodiment and Examples are merely examples, and,if necessary, a value, structure, construction, form, or materialdifferent from the above may be used.

Specifically, for example, the form of the fuel cell or fuel cartridge32 may be a form different from that mentioned in the above embodimentor Examples.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope of the present subjectmatter and without diminishing its intended advantages. It is thereforeintended that such changes and modifications be covered by the appendedclaims.

1. An apparatus comprising a fuel cell for generating electric power bydecomposing a fuel using an enzyme, wherein the fuel is a gelatinizedstarch.
 2. The apparatus according to claim 1, wherein the enzymecomprises a decomposing enzyme for promoting decomposition of thegelatinized starch to form a monosaccharide, and an oxidase forpromoting oxidation of the monosaccharide formed to decompose themonosaccharide.
 3. The apparatus according to claim 2, wherein theenzyme comprises a coenzyme oxidase for changing a coenzyme reduced dueto the oxidation of the monosaccharide to an oxidant and givingelectrons to a negative electrode through an electron mediator.
 4. Theapparatus according to claim 3, wherein the oxidant of the coenzyme isNAD⁺ and the coenzyme oxidase is diaphorase.
 5. The apparatus accordingto claim 3, wherein in that the coenzyme oxidase, the coenzyme, and theelectron mediator are immobilized on the negative electrode.
 6. Theapparatus according to claim 5, wherein the oxidase is immobilized onthe negative electrode.
 7. The apparatus according to claim 5, whereinthe oxidase and the decomposing enzyme are immobilized on the negativeelectrode.
 8. The apparatus according to claim 7, wherein thegelatinized starch is immobilized on the negative electrode.
 9. Theapparatus according to claim 5, wherein the immobilization uses animmobilizer comprising glutaraldehyde and poly-L-lysine.
 10. Anelectronic device comprising a fuel cell, wherein the fuel cellgenerates electric power by decomposing a fuel using an enzyme, whereinthe fuel is a gelatinized starch.
 11. A movable body comprising a fuelcell, wherein the fuel cell generates electric power by decomposing afuel using an enzyme, wherein the fuel is a gelatinized starch.
 12. Apower generation system comprising a fuel cell, wherein the fuel thecell generates electric power by decomposing a fuel using an enzyme,wherein the fuel is a gelatinized starch.
 13. A cogeneration systemcomprising a fuel cell, wherein the fuel cell generates electric powerby decomposing a fuel using an enzyme, wherein the fuel is a gelatinizedstarch.